Using heparin-Sepharose affinity chromatography (HSAC), we have isolated a highly enriched preparation of an epithelial cell mitogen from the conditioned media of M426 fibroblasts (derived from embryonic human lung tissue). Other cell lines are being screened for this mitogenic activity. The factor, which appears to be distinct from any previously characterized mitogen, can stimulate DNA synthesis in responsive cells (BALB/MK) at an estimated concentration of 0.1 ng/ml. Determination of an amino terminal protein sequence should be forthcoming and the generation of monoclonal and polyclonal antibodies will facilitate isolation of the factor's cDNA. Another epithelial cell mitogen has been partially purified from a commercial source of bovine pancreatic ribonuclease type 1. It is retained on HSAC but elutes at a different position than the mitogen from M426. Heparin, itself, is an inhibitor of DNA synthesis in BALB/MK cells.